There are known various kinds of heat sensitive recording (copying) systems for transmitted information wherein a color forming reaction between a colorless or pale colored basic chromogenic material and an electron accepting organic or inorganic color developing material (hereinafter referred to as "acceptor") is utilized, and heat or electric energy serves as a medium to obtain expected color images.
Now, there has been made remarkable improvements in the speed of the recording functions of thermal type facsimiles and printers. In fact, it becomes possible to record a quantity of an A-4 size paper within 20 seconds for the thermal type facsimile and a quantity of 120 characters within a second for the printer.
Following such improvements in the thermal type facsimiles and the printers, there is an increased demand for a heat sensitive recording sheet to be such that can sufficiently fit with their high speed functions and of possessing a sufficient durability without discoloration of the once developed color images even in a long storage.
However, for known heat sensitive recording sheet, in addition to that it can not sufficiently satisfy said demand there still exist defects that the once developed color images thereon is sooner or later discolored on contact with a plastic film which is used to store it in a file or the remaining blank (background) parts of the sheet are significantly apt to be undesirably fogged during its storage in the state of being piled up together with a diazo type copying paper particularly soon after having been copied.
Therefore, there is an additional increased demand for a heat sensitive recording sheet to be such that is not accompanied with the above problems.